The present invention relates to radiation curable overprint varnishes (OPV). The overprint varnishes are based on multifunctional, uncrosslinked, liquid Michael addition resins formed from the reaction of acrylate monomers, known as Michael addition reaction acceptors (hereinafter “Michael acceptors”) and β-ketoesters (e.g., ethyl acetoacetate), β-diketones (e.g., 2,4-pentanedione), and/or β-keto amides (e.g., acetoacetanilide, acetoacetamide), or other β-dicarbonyl compounds and mixtures thereof, known as Michael addition reaction donors (hereinafter “Michael donors”) that can participate in the Michael addition reaction. The OPV formulations based on the above multifunctional, uncrosslinked, liquid Michael addition resins can be cured under standard UV cure conditions with no photoinitiator or substantially less photoinitiator than is currently used in UV-curable OPV compositions.
UV-curable OPVs are known. Typically, UV-curable overprint varnish coatings comprise a composition capable of curing when exposed to UV radiation such as an acrylate monomer, oligomer or polymer in the presence of a photoinitiator of some sort. In addition to the curable component, the OPVs contain various additives to modify and improve the performance of the cured coating. Examples of OPVs include the composition disclosed in U.S. Pat. Nos. 4,204,010, 4,227,979 and published US application 20020121631. U.S. Pat. No. 4,204,010 discloses an ethylenically unsaturated reactive thixotropic agent for use in radiation-curable overprint varnishes formed by reacting a hydroxyl-containing fatty acid ester with an ethylenically unsaturated isocyanate. The composition generally contains a photoinitiating component such as benzophenone. U.S. Pat. No. 4,227,979 discloses UV-curable OPVs containing an amine acrylate and various photo-promoters. Published US application 20020121631 discloses UV-curable OPV compositions that generally include di- and trifunctional acrylate monomer, a photoinitiator, an acrylated oligomer and an acrylic polymer emulsion.
Overprint varnishes are used to produce cured coatings that provide both a protective layer and embellished feel or appearance to printed materials. High quality overprint varnish coatings can give improved rub and scuff resistance and the lower coefficients of friction necessary for use in high speed packaging lines. OPVs can also be used to improve the appearance of conventional solvent and water-based inks which are often characterized by low gloss. Special finishes can be built into OPVs by using suitable additives. For example, pearlescent effects can be achieved by using specialized mica-based pearlescent pigments. Fluorescent and optical brightening additives can also be added. There is a wide scope for specialized finishes made possible by using such additives to enhance the final cured product.
The mode of application of OPVs is dependent on the final viscosity of the uncrosslinked liquid formulation. Low viscosity formulations are typically applied using flexographic, gravure, roll-coater, and flood or curtain-coater equipment. Historically, varnishing was often carried out separately from printing. Today, many new printing presses have in-line varnish coaters fitted after the printing units. For application over UV-based inks, some presses are fitted with interdeck and pre-coater UV lamps to ensure that the inks are cured before the varnish is applied, thus achieving a smooth lay-down and high gloss.
Typical starting point formulations for standard UV-curable OPVs contain up to 10 parts per hundred (10% w/w) of a photoinitiator package. Traditional photoinitiators (e.g., benzophenone) can be toxic, expensive, and malodorous and contribute to film color, which limits applicability of varnishes over white and light-colored inks.
The amount of photoinitiator added to OPV formulations can be significantly reduced by using the acrylate oligomer technology described in patents U.S. Pat. Nos. 5,945,489 and 6,025,410 (both Ashland, Inc.) the contents of which are incorporated herein by reference. These patents disclose uncrosslinked resins prepared via the Michael addition reaction of Michael donors such as β-dicarbonyl compounds with Michael acceptors such as multifunctional acrylates. The invention disclosed herein demonstrates the advantageous use of these uncrosslinked resins alone or modified by reaction/blending with additional materials in formulations for overprint varnish applications. These additional materials include a variety of reactive diluents and adhesion promoting acrylic monomers and oligomers as well as other vinyl monomers like N-vinyl caprolactam, primary, secondary and tertiary amines, acid-functional materials, siloxane-based defoamers, wetting agents, flow and leveling aids, elastomers, waxes and other components to modify and improve performance of the varnish.
Varnishes based on the resins described above can be cured by all methods typically used to crosslink acrylic materials. Cure, or crosslinking, is usually accomplished through a free radical chain mechanism, which may require any of a number of free radical-generating species such as peroxides, hydroperoxides, REDOX combinations, etc., which decompose to form radicals when heated, or at ambient temperature in the presence of amines and transition metal promoters. Ultra violet (UV) radiation is another means of initiating reaction by decomposing an appropriate photoinitiator to form free radicals. Electron beam (EB) radiation can also be used to effect cure.
OPVs based on the novel acrylate oligomers described in this invention offer significant advantages over varnishes based on traditional multifunctional acrylic monomers and oligomers in that they can be cured by exposure to UV radiation with no photoinitiator or a fraction of the photoinitiator required for standard UV-cure varnishes. Under typical UV curing conditions for OPVs (<300 mJ/cm2 exposure), these varnishes can be effectively cured on a variety of substrates with very little or no photoinitiator.
The novel OPV formulations disclosed here exhibit performance properties that make them very effective across a range of substrates and these properties can be modified greatly depending upon oligomer composition and coating formulation rather than by blending with additives, as is done in traditional UV-curable systems. The varnishes can exhibit wide ranges of flexibility, stain resistance, scratch resistance, weather resistance, solvent resistance, etc. Almost any desired varnish performance parameter can be attained by proper selection of the raw material building blocks used to make the oligomers that form the basis of the OPV formulation.